This invention relates to shape deposition manufacturing of products having a complex configuration from using a liquid resin formulation that may be deposited in layers, polymerized (preferably photopolymerized) and precision machined.
Shape Deposition Manufacturing (SDM) technology comprises fabrication of parts or products by the sequential deposition, solidification, and precision Computer Numeric Controlled (CNC) machining of wax layers which are deposited upon one another until a desired product mold results. See e.g., U.S. Pat. Nos. 5,207,371; 5,286,573; 5,301,415 and 5,301,863. A liquid resin (i.e., polyurethane, epoxy, or ceramic gelcasting slurry) is subsequently cast into the wax mold and cured to produce a desired polymer or green ceramic part or product having a complex shape.
Unfortunately, many parts or products have complex geometries (i.e., internal chambers or overhang features) that make it difficult to accurately fabricate wax molds in successive layers without causing slumpage and remelting of previously deposited and CNC machined wax layer features. In particular, the highly fluid nature of the molten wax coupled by its slow solidification rate often causes a newly deposited layer of wax to slump considerably prior to its solidification on an adjacent or previously deposited layer. This can detract from product dimensional accuracy of the wax mold and thus the product will have imperfections. The problem of casting or molding complex parts or products using a layer by layer wax technique to form the complex mold has perplexed those of skill in the art.
Briefly, in accordance with the present invention, one is able to prepare accurate parts or products by depositing and photocuring liquid resin layers within each solidified and machined wax layer prior to the deposition of a subsequent wax layer. In other words, parts are fabricated by the deposition, solidification, and machining of successive wax and cured resin layers until the final mold encapsulating a resin formulated part results. The wax and resin layer solidification is accomplished by cooling and UV photopolymerization (e.g., photocuring) respectively. By following the described steps, a completed wax mold results with an internal core or cavity filled with a photocured resin product. The photocured resin product has the dimensions of the final desired molded part or product. Thus, the resin core may be washed from the mold by immersion in a neutral or weakly alkaline aqueous solution (preferably a solution having a pH of greater than about 8).
The resin which is preferred for use in the process is composed of at least one monofunctional water soluble vinyl or acrylic monomer (e.g., N-Vinyl pyrrolidone or N,Nxe2x80x2-Dimethylacrylamide) combined with a low molecular weight aliphatic polymer having acrylic acid (AA) or methacrylic acid (MA) functionality. A major advantage of the cured resin of the present invention is that it has dimensional stability even after exposure to high ambient humidity, yet readily dissolves in aqueous solutions. This provides an attractive means for fabricating fugitive parts or products, support materials, or soluble cores for wax molds constructed using Shape Deposition Manufacturing technology.
An object of the present invention is to provide formulations that have the ability to photocure as very thick layers, e.g. a 3 millimeter resin layer, as opposed to thin wax layers (e.g., 100 micronmeter wax layers typically encountered in accordance with conventional Shape Deposition Manufacturing technology) over a 2 minute period in a single pass using ultraviolet (UV) radiation.
Another object of the present invention is to provide techniques and formulations that enable casting or molding parts and products that have smooth surfaces and with minimal chipping.
A further objection of the present invention is to provide techniques and formulations that provide the ability to fabricate positive image structures.
Another object of the present invention is to provide techniques and formulations that have the ability to fill fine crevices and machine features in a wax mold as it is being manufactured.
A further object of the present invention is to provide techniques and formulations that result in higher strength molds than conventional Shape Deposition Manufacturing technology.
Yet another object of the present invention is to provide techniques and formulations that enable manufacture of a completely soluble, organic molded product with no mineral filler.
Additional objects of the present invention include the provision of techniques and formulations for making molds that incorporate the use of sucrose, which reduces shrinkage and reduces exotherm (i.e., energy) on curing, resulting in less bubbling and smoother surfaces in the part and/or mold. Notably, the refractive index of sucrose is similar to the resin monomers of the present invention. Further, it has been discovered that sucrose has minimal effect upon UV transmission and resin curing.